There is a clear trend in the electronics industry towards the use of surface mount components, that is, components which are mounted with their leads, or pads, directly in contact with each of a plurality of metallized areas on a major surface of a circuit board. To solder bond such surface mount components to a circuit board, a volume of solder paste is deposited (printed) onto the metallized areas on the board which are arranged in a pattern corresponding to that of the leads/pads on the component. Following deposit of the soldering paste, each component is placed on the circuit board so that its leads/pads are in contact with the corresponding solder paste-coated metallized areas on the circuit board. Thereafter the solder paste is reflowed, typically by heating the circuit board, either in an oven, or by other means, to bond the components to the board.
Presently, "RMA" type solder paste is generally used to solder bond surface mount components. This type of paste generally is comprised a rosin-based flux and solder powder which are dissolved in a solvent together with one or more rheology-modifying (thixotropic) agents. While RMA-type pastes are widely used, such pastes commonly leave rosin residues on the circuit board following paste reflow. Often, such residues must be removed either for cosmetic reasons or to improve testability of the board. Unfortunately, the rosin residues remaining after paste reflow are extremely difficult to remove unless aggressive solvents (typically, undesirable cholorfluorocarbons, or halogenated hydrocarbons) are used.
In an effort to eliminate the difficulty in cleaning residues remaining from the use of RMA pastes, water-soluble solder pastes, such as taught in U.S. Pat. Nos. 4,460,414 to J. S. Hwang, and 4,872,928 issued to W. Jacobs (herein incorporated by reference), are now being offered. As their name implies, water-soluble pastes are soluble in water, allowing post-solder cleaning of the solder paste residues left on a circuit board with water, or a mixture of water and a mild detergent. Water-soluble solder pastes differ from the RMA pastes in that they contain a water-soluble flux, typically an acid and/or amine activator carried by a glycol vehicle, in place of a rosin-based flux.
While water-soluble pastes eliminate the need to employ aggressive solvents to remove paste residues after soldering, water-soluble fluxes incur a disadvantage of their own. The water-soluble flux within current day water-soluble pastes invariably contains a glycol vehicle. During paste reflow, the circuit board, which is basically a polymer matrix, is typically heated about its glass transition temperature T.sub.g, permitting the glycol molecules of the flux within the paste to diffuse into the surface of the circuit board. Once the circuit board cools, the glycol molecules become trapped in the board surface.
The trapped glycol molecules, which are hydrophillic, attract water molecules in the environment to the board surface. The water molecules attracted to the board surface tend to reduce the surface insulation resistance (SIR) of the board which is undesirable as it may adversely affect the input impedance of certain electronic components placed thereon. For this reason, many electronics manufacturers have not readily adopted water-soluble pastes, especially for applications demanding a high SIR.
Thus, there is a need for a water-soluble soldering paste whose residues may be cleaned with water and/or detergent, but which maintains a high surface insulation resistance.